WASHINGTON — North Korea announced Tuesday that it intended to conduct its first nuclear test, prompting warnings from Tokyo to Washington that an underground explosion would lead to a sharp response and could undermine the security balance in Asia.

The North did not say when it would attempt to test a weapon, and experts inside and outside the Bush administration said the announcement itself is a negotiating ploy, intended to force the White House into lifting economic sanctions and conducting one-on-one talks with the isolated country.

American intelligence officials said they saw no signs that a test was imminent. But they cautioned that two weeks ago, American officials who have reviewed recent intelligence reports said American spy satellites had picked up evidence of indeterminate activity around North Korea's main suspected test site. It was unclear to them whether that was part of preparations for a test, or perhaps a feint related to the visit at that time to Washington of South Korea's president, Roh Moo Hyun.

At that meeting, Bush and Roh discussed the possibility of a test, and Roh said the event would "change the nature" of South Korea's policy of economic engagement with the North, Roh told Americans he met afterward.

But they did not appear to have a coordinated strategy, and a senior Asian diplomat in Washington said Tuesday "no one is quite sure how to respond" if the North conducts a test in coming weeks or months.

In public, the Bush administration's response was muted on Tuesday and left the American response as unclear as the North Korean threat.

North Korea has long possessed plutonium fuel needed to manufacture nuclear weapons, and American intelligence agencies believe the country expanded its fuel stockpile in recent years so that it could now manufacture roughly six to eight weapons, and perhaps more. That inventory was increased, the North says, since it evicted international inspectors in early 2003, just as the Bush administration was focused on the Iraq invasion.

The North claimed more than a year ago that it possessed a "nuclear deterrent," but the absence of a test has created a convenient diplomatic ambiguity, allowing China to raise doubts about how far the country has come and giving Washington room after President Bush's declaration in the first term that he would never "tolerate" a nuclear armed North Korea.

It is unclear whether the North Koreans have now determined that ambiguity is no longer in their interests. In a statement issued Tuesday on KCNA, the North's official news outlet, the country's foreign ministry said that "the U.S. extreme threat of a nuclear war and sanctions and pressure compel the DPRK to conduct a nuclear test, an essential process for bolstering nuclear deterrent, as a self-defense measure in response."

But earlier this month a North Korean general, Ri Chan Bok, told a visiting American expert, Selig Harrison, that no test was necessary.

"If we have an underground test, it could have radioactive leakage," Harrison, who has been visiting the country for three decades, quoted the general as saying last week. "These rumors are spread by U.S. agencies to smear us. I have never heard indications of a nuclear test in our government or armed forces."

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In a statement, Frederick Jones, the spokesman for the National Security Council, said a test would "severely undermine our confidence in North Korea's commitment to denuclearization" and "pose a threat to peace and security in Asia and the world."

"A provocative action of this nature would only further isolate the North Korean regime and deny the people of the North the benefits offered to them" in six-nation talks that have not reconvened in more than a year, the White House statement said.

But behind closed doors, the announcement touched off a flurry of meetings, as officials wrestled with uncertain intelligence, questions about whether China or South Korea could prevent a test, and the possibility that a test could take place before the elections.

The American statement did not set forth the lines in the sand that marked the North Korean nuclear standoffs of the 1990's, when the Clinton administration began reinforcing American forces on the Korean peninsula in response to a threat by the North to convert its supply of spend nuclear reactor fuel into bombs. Clinton's advisers recommended at the time that if the North began to move that supply to a facility where it could be fashioned into bomb fuel, the president should order an airstrike to destroy the facility. Clinton never had to face that choice.

But American officials, declining to be identified because they are not authorized to speak about North Korean policy, have said in recent weeks that the administration assumed that sooner or later, the North would conduct a test. "You could argue that it wouldn't be an all-bad thing," one Administration hawk said recently, "because it would finally unify the Chinese and the Russians and the South Koreans," all of whom have been reluctant to pressure the North.

Michael Green, who handled North Korea issues for the National Security Council until he left the White House last year, said that "the evidence has grown, especially with the missile launch, that North Korea has its own escalation ladder, and they would agree to postpone a test only for the right price." He thought it unlikely that price would be met, and said "the North has calculated that they can take the heat from China and Japan, and they are not losing much from South Korea anyway."

In Tokyo, North Korea's sudden announcement was the first international test to face Japan's newly inaugurated prime minister, Shinzo Abe, a nationalist who has vowed to make security a top priority. Abe warned Pyongyang against the test in stern terms rarely seen in the cautious language of Japanese diplomacy.

"Japan and the world absolutely will not tolerate a nuclear test," he told reporters, in a statement worded more sharply than the Bush administration's. "The international community would respond harshly."

Secretary of State Condoleezza Rice told reporters in Cairo, where she met with her counterparts from several Arab nations on regional issues, including Iran's nuclear program, that the North's announcement was disturbing and that a nuclear test would be "a very provocative act by the North Koreans."